This invention relates to the manufacture of hydrofluorocarbons (i.e., HFCs) and azeotropic compositions thereof, and more particularly to the production of hexafluoropropanes and their azeotropic compositions with HF.
1,1,1,2,3,3-hexafluoropropane (i.e., CHF2CHFCF3 or HFC-236ea) and 1,1,1,3,3,3-hexafluoropropane (i.e., CF3CH2CF3 or HFC-236fa) are useful as refrigerants, fire extinguishants, heat transfer media, gaseous dielectrics, sterilant carriers, polymerization media, particulate removal fluids, carrier fluids, buffing and abrasive agents, displacement drying agents and power cycle working fluids. In particular, HFC-236ea and HFC-236fa are highly effective as refrigerants for use in refrigeration equipment.
U.S. Pat. No. 5,171,901 discloses a process for the preparation of HFC-236fa by contacting a mixture of hexachloropropene and HF with a catalyst consisting of a mixture of CrCl3 and MgF2 at temperatures ranging from 350xc2x0 C. to 500xc2x0 C. The reaction temperatures and yields of HFC-236fa were as follows: 350xc2x0 C., none detected; 400xc2x0 C., 10%; 450xc2x0 C., 55%; and 500xc2x0 C., 64%. Other products formed in varying amounts were CF3CHClCF3, CF3CCl2CF3, CF3CClxe2x95x90CF2, CF3CClxe2x95x90CClF, and CF3CClxe2x95x90CCl2. HFC-236ea has been prepared by the hydrogenation of hexafluoropropane. There is an interest in developing additional, efficient processes for the manufacture of HFC-236ea and HFC-236fa from various starting materials.
This invention provides a process for producing CF3CHFCHF2. This process comprises reacting CF3CFxe2x95x90CHF with HF at an elevated temperature over a catalyst selected from the group consisting of aluminum fluoride, fluorided alumina, metals supported on aluminum fluoride, metals supported on fluorided alumina and catalysts comprising trivalent chromium. Also provided is a process for enriching CF3CFxe2x95x90CHF from a starting mixture containing CF3CFxe2x95x90CHF and CF3CHxe2x95x90CF2. This process comprises reacting said starting mixture with HF at an elevated temperature over a catalyst consisting essentially of carbon, a catalyst consisting essentially of metal halides supported on carbon, a catalyst consisting essentially of Cr2O3, or mixtures thereof (provided that when Cr2O3 is present the temperature is about 250xc2x0 C. or less) to produce a product mixture containing CF3CH2CF3 and unreacted CF3CFxe2x95x90CHF wherein the mole ratio of CF3CFxe2x95x90CHF to CF3CHxe2x95x90CF2 is greater than the ratio thereof in the starting mixture. This invention provides for production of CF3CH2CF3 (or both CF3CFHCHF2 and CF3CH2CF3) by reacting a starting mixture containing both CF3CFxe2x95x90CHF and CF3CHxe2x95x90CF2 with HF at an elevated temperature over a fluorination catalyst.
This invention further provides compositions which consist essentially of hydrogen fluoride in combination with an efective amount of a compound selected from the group consisting of CF3CHFCHF2 and CF3CH2CF3 to form an azeotrope or azeotrope-like composition with hydrogen fluoride, said composition containing from about 31 to 60 mole percent CF3CHFCHF2 or from about 41 to 63 mole percent CF3CH2CF3.
The addition of hydrogen fluoride across the double bond of olefinic compounds normally follows Markovnikov""s rule (i.e., hydrogen is added to the carbon atom of the double bond which has a hydrogen atom, and fluorine to the olefinic carbon atom which has a fluorine atom). U.S. Pat. No. 5,268,122 provides examples of Markovnikov addition to olefinic fluoro-carbon bonds. In accordance with this invention, HF adds across the double bond of CF3CFxe2x95x90CHF to produce CH2CHFCF3 rather than the Markovnikov expected product, CH2FCF2CF3. Thus, this invention provides a process which involves reacting CF3CFxe2x95x90CHF with HF to add H at the olefinic carbon different from the olefinic carbon already containing a hydrogen such that CF3CFHCHF2 is produced.
CF3CFxe2x95x90CHF can be prepared according to R. N. Hazeldine et al., J. Chem. Soc. Perkin Trans.1, 1303-07 (1974). CF3CFxe2x95x90CHF may also be produced from CF3CF2CH2F (i.e., HFC-236cb) by dehydrofluorination (e.g., by reacting CF3CF2CH2F with KOH). Because of this unexpected chemistry, HFC-236cb can be converted to HFC-236ea by first dehydrofluorinating HFC-236cb to CF3CFxe2x95x90CHF and then reacting the CF3CFxe2x95x90CHF with HF to afford HFC-236ea. Accordingly, this invention provides a process for producing CF3CHFCHF2 from its isomer CF3CF2CH2F by dehydrofluorinating CF3CF2CH2F to CF3CFxe2x95x90CHF, and reacting CF3CFxe2x95x90CHF with HF to provide CF3CHFCHF2.
Preferred catalysts for the reaction of CF3CFxe2x95x90CHF with HF include aluminum fluoride, fluorided alumina, metals supported on aluminum fluoride, metals supported on fluorided alumina and catalysts containing trivalent chromium. Suitable metals (including metal oxides, metal halides and/or other metal salts) for use on aluminum fluoride or fluorided alumina include (in addition to trivalent chromium) Group VIII metals (iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum), Group VIIB metals (manganese, rhenium), Group IIIB metals (scandium, yttrium, lanthanum), Group IB metals (copper, silver, gold), zinc and/or metals having an atomic number of 58 through 71 (cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetium). Preferably the total metal content of the catalyst will be from about 0.1 to 20 percent by weight; typically from about 0.1 to 10 percent by weight of the supported catalyst. It is understood that in catalyst preparation a metal compound may be supported on an alumina, and the resulting supported metal composition may then be fluorinated. Especially preferred for the reaction of CF3CFxe2x95x90CHF are chrome oxide catalysts prepared by the pyrolysis of ammonium dichromate (see U.S. Pat. No. 5,036,036 for preparative details).
Typically the temperature for the reaction of CF3CFxe2x95x90CHF is from about 275xc2x0 C. to 450xc2x0 C. Temperatures between 300xc2x0 C. and 400xc2x0 C. are generally preferred.
A process for production of CF3CH2CF3 is also provided by this invention. CF3CH2CF3 may be produced by reacting CF3CHxe2x95x90CF2 in a starting mixture containing CF3CHxe2x95x90CF2 and CF3CFxe2x95x90CHF with HF to add H at the olefinic carbon already containing a hydrogen. Fluorination catalysts are used for this reaction. Preferably, the mole ratio of CF3CHxe2x95x90CF2 to CF3CFxe2x95x90CHF in the starting mixture is from 5:95 to 95:5.
Suitable catalysts which can be used for reacting CF3CHxe2x95x90CF2 with HF to produce CF3CH2CF3 include vapor phase fluorination catalysts. Catalysts which may be used in accordance with this invention include metals (including metal oxides, metal halides and/or other metal salts); fluorided alumina; aluminum fluoride; metals on aluminum fluoride; metals on fluorided alumina; metals on carbon; and chromium catalysts. Suitable metals for use in such catalysts include chromium, Group VIII metals (iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum), Group VIIB metals (manganese, rhenium), Group IIIB metals (scandium, yttrium, lanthanum), Group IB metals (copper, silver, gold), zinc and/or metals having an atomic number of 58 through 71 (cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetium). Preferably, when used on a support, the total metal content of the catalyst will be from about 0.1 to 20 percent by weight; typically from about 0.1 to 10 percent by weight.
Fluorided alumina and aluminum fluoride can be prepared as described in U.S. Pat. No. 4,902,838. Metals on aluminum fluoride and metals on fluorided alumina can be prepared by procedures described in U.S. Pat. No. 4,766,260. Catalysts comprising chromium are well known in the art, e.g., see U.S. Pat. No. 5,036,036. Chromium supported on alumina can be prepared as described in U.S. Pat. No. 3,541,165. Chromium supported on carbon can be prepared as described in U.S. Pat. No. 3,632,834. Preferred catalysts include aluminum fluoride, fluorided alumina, catalysts comprising carbon and catalysts comprising chromium oxide.
Typically, the temperature for the reaction of CF3CHxe2x95x90CF2 is from about 150xc2x0 C. to 450xc2x0 C. Temperatures between 175xc2x0 C. and 400xc2x0 C. are generally preferred. It is noted that suitable catalysts include those listed above for the reaction of CF3CFxe2x95x90CHF with HF. When these catalysts are used at sufficiently high temperatures, the product contains CF3CFHCHF2 along with CF3CH2CF3.
In accordance with this invention, catalysts selected from the group consisting of (1) catalysts consisting essentially of carbon, (2) catalysts consisting essentially of metal halides supported on carbon, (3) catalysts consisting of Cr2O3, and mixtures thereof, may be used such that CF3CHxe2x95x90CF2 is selectively reacted with HF and the reaction product contains CF3CH2CF3 and unreacted CF3CFxe2x95x90CHF from the starting mixture. Suitable metal halides include chlorides and fluorides of chromium, nickel, cobalt, zinc, copper, iron and/or manganese. Especially preferred are carbon catalysts which have an ash content of less than about 0.1 percent by weight (see U.S. Pat. No. 5,136,113 for preparative details). The selective reaction of CF3CHxe2x95x90CF2 with HF may be used to provide enriched CF3CFxe2x95x90CHF from a mixture of CF3CFxe2x95x90CHF and CF3CHxe2x95x90CF2. The selective reaction of CF3CHxe2x95x90CF2 is normally conducted at a temperature within the range of about 150xc2x0 C. to 350xc2x0 C. (preferably between about 200xc2x0 C. and 300xc2x0 C.); except that when Cr2O3 is present the temperature should normally be about 250xc2x0 C. or less.
If desired, the CF3CH2CF3 produced by selective reaction of CF3CHxe2x95x90CF2 may be separated by conventional processes (e.g., distillation) from the unreacted CF3CFxe2x95x90CHF, and the CF3CFxe2x95x90CHF may then be reacted with HF as described above to form CF3CHFCHF2 in an additional step. Accordingly, a process for producing CF3CHFCHF2 is provided which also produces CF3CH2CF3 comprises reacting a starting mixture comprising CF3CHxe2x95x90CF2 and CF3CFxe2x95x90CHF with HF at an elevated temperature using a catalyst consisting essentially of carbon, a catalyst consisting essentially of metal halides supported on carbon, a catalyst consisting essentially of Cr2O3, or mixtures thereof (provided that when Cr2O3 is present the temperature is about 250xc2x0 C. or less) to provide a product mixture containing CF3CH2CF3 and unreacted CF3CFxe2x95x90CHF; separating CF3CH2CF3 from CF3CFxe2x95x90CHF in the product mixture; and reacting CF3CFxe2x95x90CHF from the product mixture with HF at an elevated temperature over a catalyst selected from the group consisting of aluminum fluoride, fluorided alumina, metals on aluminum fluoride, metals on fluorided alumina and catalysts containing trivalent chromium to produce CF3CHFCHF2. CF2xe2x95x90CHCF3 can be prepared according to P. Tarrant et al., J. Am. Chem. Soc., 77, 2783-7 (1955).
It is noted that at temperatures of about 250xc2x0 C. or less, Cr2O3 catalyst may be advantageously used for the selective reaction of CF3CHxe2x95x90CF2 from a mixture containing CF3CHxe2x95x90CF2 and CF3CFxe2x95x90CHF to provide a product mixture containing CF3CH2CF3 and unreacted CF3CFxe2x95x90CHF; whereas at higher temperatures Cr2O3 catalyst more readily provides reaction of both CF3CHxe2x95x90CF2 and CF3CFxe2x95x90CHF. Accordingly, two reaction zones sequentially operating at a temperature of about 250xc2x0 C. or less, and at a temperature above 250xc2x0 C. may be used to sequentially convert CF3CHxe2x95x90CF2 and then CF3CFxe2x95x90CHF.
The molar ratio of HF to the C3HF5 olefin or mixture of olefins being reacted typically ranges from about 1:1 to about 100:1, and is preferably within the range of about 1:1 to about 4:1. The contact time is typically from about 1 to about 100 seconds.
1,1,1,3,3,3-Hexafluoropropane and/or 1,1,1,2,3,3-hexafluoropropane may be recovered from the reaction products by using conventional techniques such as decantation and distillation.
The process of this invention can be carried out readily in the vapor phase using well known chemical engineering practice.
The reaction zone and its associated feed lines, effluent lines and associated units should be constructed of materials resistant to hydrogen fluoride. Typical materials of construction, well-known to the fluorination art, include stainless steels, in particular of the austenic type, the well-known high nickel alloys, such as Monel(copyright) nickel-copper alloys, Hastelloy(copyright) nickel-based alloys and, Inconel(copyright) nickel-chromium alloys, and copper-clad steel. Also suitable for reactor fabrication are such polymeric plastics as polytrifluorochloroethylene and polytetrafluoroethylene, generally used as linings.
The present invention also provides compositions which consist essentially of hydrogen fluoride and an effective amount of a compound selected from CF3CH2CF3 and CHF2CHFCF3 to form an azeotropic combination with hydrogen fluoride. By effective amount is meant an amount which, when combined with HF, results in the formation of an azeotrope or azeotrope-like mixture. As recognized in the art, an azeotrope or an azeotrope-like composition is an admixture of two or more different components which, when in liquid form under given pressure, will boil at a substantially constant temperature, which temperature may be higher or lower than the boiling temperatures of the individual components, and which will provide a vapor composition essentially identical to the liquid composition undergoing boiling.
An azeotrope is a liquid mixture that exhibits a maximum or minimum boiling point relative to the boiling points of surrounding mixture compositions. An azeotrope is homogeneous if only one liquid phase is present. An azeotrope is heterogeneous if more than one liquid phase is present. Regardless, a characteristic of minimum boiling azeotropes is that the bulk liquid composition is then identical to the vapor composition in equilibrium therewith, and distillation is ineffective as a separation technique. For the purpose of this discussion, azeotrope-like composition means a composition which behaves like an azeotrope (i.e., has constant-boiling characteristics or a tendency not to fractionate upon boiling or evaporation). Thus, the composition of the vapor formed during boiling or evaporation of such compositions is the same as or substantially the same as the original liquid composition. Hence, during boiling or evaporation, the liquid composition, if it changes at all, changes only to a minimal or negligible extent. This is to be contrasted with non-azeotrope-like compositions in which during boiling or evaporation, the liquid composition changes to a substantial degree.
Accordingly, the essential features of an azeotrope or an azeotrope-like composition are that at a given pressure, the boiling point of the liquid composition is fixed and that the composition of the vapor above the boiling composition is essentially that of the boiling liquid composition (i.e., no fractionation of the components of the liquid composition takes place). It is also recognized in the art that both the boiling point and the weight percentages of each component of the azeotropic composition may change when the azeotrope or azeotrope-like liquid composition is subjected to boiling at different pressures. Thus an azeotrope or an azeotrope-like composition may be defined in terms of the unique relationship that exists among components or in terms of the compositional ranges of the components or in terms of exact weight percentages of each component of the composition characterized by a fixed boiling point at a specified pressure. It is also recognized in the art that various azeotropic compositions (including their boiling points at particular pressures) may be calculated (see, e.g., W. Schotte, Ind. Eng. Chem. Process Des. Dev. 1980, 19, pp 432-439). Experimental identification of azeotropic compositions involving the same components may be used to confirm the accuracy of such calculations and/or to modify the calculations for azeotropic compositions at the same or other temperatures and pressures.
Compositions may be formed which consist essentially of azeotropic combinations of hydrogen fluoride with a compound selected from CF3CH2CF3 and CHF2CHFCF3. These include a composition consisting essentially of from about 40 to about 69 mole percent HF and from about 31 to 60 mole percent CHF2CHFCF3 (which forms an azeotrope boiling at a temperature from between about xe2x88x9225xc2x0 C. and about 100xc2x0 C. and a pressure between about 32 kPa and about 2500 kPa); and a composition consisting essentially of from about 37 to about 59 mole percent HF and from about 41 to about 63 mole percent CF3CH2CF3 (which forms an azeotrope boiling at a temperature between about xe2x88x9225xc2x0 C. and 100xc2x0 C. and a pressure between about 44 kPa and about 2900 kPa).
At atmospheric pressure, the boiling points of hydrofluoric acid, HFC-236ea and HFC-236fa are about 19.5xc2x0 C., 6xc2x0 C. and xe2x88x920.7xc2x0 C., respectively. However, the relative volatility at 234 kPa (34 psia) and 20xc2x0 C. of HF and HFC-236ea was found to be nearly 1.0 as 56 mole percent HF and 44 mole percent HFC-236ea was approached; and the relative volatility at 294 kPa (42.7 psia) and 20xc2x0 C. of HF and HFC-236fa was found to be nearly 1.0 as 50 mole percent HF and 50 mole percent HFC-236fa was approached. These data indicate that the use of conventional distillation procedures will not result in the separation of a substantially pure compound because of the low value of relative volatility of the compounds.
To determine the relative volatility of HF with each of HFC-236ea and HFC-236fa, the so-called PTx Method was used. In this procedure, the total absolute pressure in a cell of known volume is measured at a constant temperature for various known binary compositions. Use of the PTx Method is described in greater detail in xe2x80x9cPhase Equilibrium in Process Designxe2x80x9d, Wiley-Interscience Publisher, 1970, written by Harold R. Null, on pages 124 to 126.
These measurements can be reduced to equilibrium vapor and liquid compositions in the cell by an activity coefficient equation model, such as the Non-Random, Two-Liquid (NRTL) equation, to represent liquid phase non-idealities. Use of an activity coefficient equation, such as the NRTL equation, is described in greater detail in xe2x80x9cThe Properties of Gases and Liquidsxe2x80x9d, 4th Edition, publisher McGraw Hill, written by Reid, Prausnitz and Poling, on pages 241 to 387; and in xe2x80x9cPhase Equilibria in Chemical Engineeringxe2x80x9d, published by Butterworth Publishers, 1985, written by Stanley M. Walas, pages 165 to 244.
Without wishing to be bound by any theory or explanation, it is believed that the NRTL equation can sufficiently predict whether or not mixtures of HF and any of HFC-236ea and HFC-236fa behave in an ideal manner, and can sufficiently predict the relative volatilities of the components in such mixtures. Thus, while HF has a good relative volatility compared to HFC-236ea at low HFC-236ea concentrations, the relative volatility becomes nearly 1.0 as 44 mole percent HFC-236ea was approached at 20xc2x0 C. This would make it impossible to separate HFC-236ea from HF by conventional distillation from such a mixture. Where the relative volatility approaches 1.0 defines the system as forming a near-azeotrope. Where the relative volatility is 1.0 defines the system as forming an azeotrope.
It has been found that azeotropes of HF and HFC-236ea are formed at a variety of temperatures and pressures. At a pressure of 34 psia (234 kPa) and 20xc2x0 C., the azeotrope vapor composition was found to be about 56 mole percent HF and about 44 mole percent HFC-236ea. Based upon the above findings, it has been calculated that an azeotropic composition of about 69 mole percent HF and about 31 mole percent HFC-236ea can be formed at xe2x88x9225xc2x0 C. and 4.7 psia (32 kPa) and an azeotropic composition of about 40 mole percent HF and about 60 mole percent HFC-236ea can be formed at 100xc2x0 C. and 363 psia (2500 kPa). Accordingly, the present invention provides an azeotrope or azeotrope-like composition consisting essentially of from about 69 to 40 mole percent HF and from about 31 to 60 mole percent HFC-236ea, said composition having a boiling point from about xe2x88x9225xc2x0 C. at 32 kPa to about 100xc2x0 C. at 2500 kPa.
It has been found that azeotropes of HF and HFC-236fa are formed at a variety of temperatures and pressures. At a pressure of 42.7 psia (about 294 kPa) and 20xc2x0 C., the azeotrope vapor composition was found to be about 50 mole percent HF and about 50 mole percent HFC-236fa. Based upon the above findings, it has been calculated that an azeotropic composition of about 37 mole percent HF and about 63 mole percent HFC-236fa can be formed at 100xc2x0 C. and 422 psia (2900 kPa). Accordingly, the present invention provides an azeotrope or azeotrope-like composition consisting essentially of from about 59 to 37 mole percent HF and from about 31 to 63 mole percent HFC-236fa, said composition having a boiling point from xe2x88x9225xc2x0 C. at about 44 kPa to about 100xc2x0 C. at about 2900 kPa.
The HFC-236ea/HF and HFC-236fa/HF azeotropes are useful as feed to produce HFC-227ca (CHF2CF2CF3) and/or HFC-227ea (CF3CHFCF3) (see U.S. Pat. No. 5,177,273). It will also be apparent to one of ordinary skill in the art that distillation including azeotropes with HF can typically be run under more convenient conditions than distillation without HF (e.g., where HF is removed prior to distillation).